Pertussis is highly transmissible and it is one of the most infectious vaccine-preventable diseases. The expected number of secondary cases caused by an infectious individual with pertussis (R0) is approximately 14, similar to measles, and several-fold greater than influenza1 (see section 1.1.1). Transmission occurs by aerosolised droplets, and the incubation period is 7 to 10 days (range 5 to 21 days).
The initial catarrhal stage, during which infectivity is greatest, is of insidious onset with rhinorrhoea and an irritating cough that can progress to severe paroxysms of coughing. In the catarrhal stage, which usually lasts one to two weeks, the only clue to diagnosis may be contact with a known case. This stage is followed by the paroxysmal stage, with coughing episodes characterised by a series of short expiratory bursts, followed by an inspiratory gasp or typical whoop, and/or vomiting. Patients appear relatively well between paroxysms and are usually afebrile.
The above description is of the clinical presentation in a non-immune child, but it does vary with age, immunisation status and previous infection. In young infants apnoea and/or cyanosis may precede paroxysmal cough, and it is important they are recognised as presenting symptoms of severe disease. Thus pertussis must be considered in infants presenting with an acute life-threatening event, or apnoea.2 In school-aged children immunised in infancy, the clinical symptoms that distinguish pertussis from other causes of coughing illnesses are inspiratory whoop, post-tussive vomiting and the absence of wheezing and fever.3
Almost all pertussis infections in adolescents and adults occur in the context of previous infection and/or immunisation. Persistent cough, not infrequently for more than four weeks, is the cardinal feature in adults.4 Studies performed in several countries during both epidemic and non-epidemic periods have shown that between 12 and 37 percent of school-aged children, adolescents and adults with persistent cough have evidence of recent B. pertussis infection.3, 5–9 A primary care-based study in New Zealand performed during the early phase of the 2011 to 2013 epidemic showed that recent B. pertussis infection was present in 17 percent of children aged 5–16 years and 7 percent of adults aged
17–49 years presenting to primary care with a persistent cough of two or more weeks’ duration.10
In adults, cough is worse at night and often paroxysmal. Adults describe being awoken by a choking sensation. Post-tussive vomiting and whoop are infrequent. A scratchy throat and sweating attacks are common.
The most common complications of pertussis are secondary infections, such as otitis media and pneumonia, and the physical sequelae of paroxysmal coughing (eg, subconjunctival haemorrhages, petechiae, epistaxes, central nervous system haemorrhages, pneumothoraces and herniae). At the peak of the paroxysmal phase vomiting can lead to weight loss, especially in infants and young children. The disease is most often severe in infants in the first few months of life. Of infants with pertussis sufficiently severe to require intensive care admission, one in six will either die or be left with brain or lung damage.11